1. Field of the Invention
The invention relates to an apparatus for shaping thermoplastic materials, comprising a suction-pressure bell, an upper clamping frame, a lower clamping frame, between which the material to be shaped is able to be clamped, and a lower part with a mould.
2. Description of the Related Art
In a known apparatus of this type, for the moulding operation, in which compressed air is introduced into the pressure bell in order to press the film material against the mould, the pressure bell and the lower part supporting the mould are pressed against each other by a hydraulically activated drive device, in order to absorb the forces, tending to separate the pressure bell and the lower part, together with the clamping frame lying in between. In the case of large parts, such as for example refrigerator inserts, very large forces, for example of 800 kN and greater, may occur. A major problem which arises in this respect is constituted not only by the very high holding forces which are necessary in order to press the pressure bell and its lower part against each other but also by the fact that only clamping frames which are essentially adapted to the length and width of the pressure bell can be used. If, on the other hand, small moulds are to be used for producing smaller parts, other clamping frames, i.e. clamping frames with a significantly smaller through-opening, must be used. Since the upper clamping frame must bear in a sealing manner against the pressure bell, this clamping frame keeps a relatively large surface area on which the pressure in the pressure bell acts, based on a pressure bell for large parts to be produced, when a changeover is made to small parts to be produced. The surface area of the clamping frame is so great because it has a small through-opening because of the smaller mould but still must reach up to the rim of the pressure bell because of the sealing. The pressure applied in the pressure bell then acts on this large surface area of the upper clamping frame, which leads to a deformation of the clamping frame because the clamping force which is transferred from the lower part via the lower clamping frame to the upper clamping frame, acts with greater leverage in the case of clamping frames with a small opening than in the case of clamping frames with a large opening. At the same time, the relatively great forces are also transferred to the seal which is provided between the pressure bell and the upper clamping frame. This seal is made in the form of a round rubber cord and can therefore not be shaped in any way desired, for example to compensate for tolerances in the thickness dimensions of the upper clamping frame and also different thicknesses of the material to be shaped. To avoid the greatest disadvantages, it is known to use stops which limit excessive force acting on the upper clamping frame and the seal when the pressure bell and the lower part are moved together. In this case, however, the overall apparatus is even more sensitive with respect to tolerances occurring at the upper clamping frame flange and with respect to the material thickness of the material to be shaped.
The object of the invention is to design an apparatus wherein the seal is not squeezed or otherwise impaired when the suction-pressure bell and the lower part are moved together, wherein the sealing effect can be adapted to the prevailing internal pressure in the suction-pressure bell and wherein it is possible to compensate for thickness tolerances at the flange of the upper clamping frame and with respect to the material to be moulded.
This object is achieved by providing a seal which can be expanded under controllable force between the suction-pressure bell and the upper clamping frame.
This design has the advantage that the upper clamping frame can be held in the moulding position by a preliminary loading of the seal. Since the seal is expandable arbitrarily by force action, which is controllable, it can be adapted in its sealing action to the respectively prevailing pressure conditions in the interior space of the suction-pressure bell. Since the seal is expandable arbitrarily under force action, it is readily possible to compensate for dimensional tolerances with respect to the thickness dimensions of the flange of the upper clamping frame and also with respect to changes in the thickness of the material to be shaped.
The advantages of this design become particularly evident if, in a further design of the invention, the suction-pressure bell and the lower part, supporting the mould, are able to be connected to each other by a force-absorbing device, which completely absorbs forces occurring during the moulding operation.
The suction-pressure bell and the assigned lower part and also the clamping frames arranged in between are held together by this force-absorbing device and are not pressed together as in the prior art. The high forces occurring in the compressive shaping are thus absorbed by the force-absorbing device, so that only significantly lower forces act, in particular on the clamping frames, than was the case when the two parts were pressed against each other, because the pressing of the suction-pressure bell and the lower part these mutual pressing forces also to be transferred via the clamping frames. If, on the other hand, the suction-pressure bell and the lower part are held together by externally applied force-absorbing devices in a way corresponding to the present invention, the two frames arranged between the suction-pressure bell and the lower part are largely free from the enormous forces. This is because the force resulting from the pressure of approximately 4 bar and the surface area of the suction-pressure bell in the separating region does not act on the clamping frame and the mould, and consequently on the lower part. Instead these forces act via the mould on the lower part, where they are absorbed by the externally arranged locking devices, so that there is largely no longer any force transfer, in particular via the clamping frames. It goes without saying that forces act on the clamping frames, in particular if they expose a significantly smaller opening than corresponds to the surface area of the suction-pressure bell in the separating region. However these forces can be absorbed by corresponding shaping of the clamping frames, because they are significantly lower than when the entire forces are transferred via the clamping frame to the lower part, as is the case in the prior art.
The term xe2x80x9csuction-pressure bellxe2x80x9d is intended to express the idea that, before the compressive shaping, the film or sheet is drawn into the suction-pressure bell by sucking action and in this way is preshaped.
The force-absorbing device, formed as a locking device, preferably has locking bars fixedly arranged on the circumference of the suction-pressure bell which, in the joined-together state of the suction-pressure bell and the lower part, are able to be fixed by coupling devices arranged on the lower part. These coupling devices may be formed as quick-clamping devices, which grasp and securely hold the ends of the locking bars. By appropriate dimensioning and selection of the necessary number of locking bars, the very high forces can be absorbed by these locking bars and their coupling devices, and no clamping forces of any great magnitude have to be applied here in order to hold together the ends of the locking bars and the parts of the coupling device.
The advantages of the design according to the invention with regard to the arrangement of an expandable seal are also achieved if the suction-pressure bell and the lower part are able to be pressed against each other by respective drive devices, in particular if the mutual pressing force is limited by stops.
The seal between the suction-pressure bell and the upper clamping frame, which was previously severely compressed by the external pressing forces, is now relieved of the great forces via the locking device. In a development of the invention, the seal is accommodated in a groove of the suction-pressure bell and is able to be pressed against the upper clamping frame by a pressure medium for sealing between the suction-pressure bell and the clamping frame. In the design according to invention, the seal is no longer subjected to the external pressing forces directed against one another but is now loaded only to the degree necessary for sealing by a pressure medium, in order to avoid a decline in the positive pressure in the suction-pressure bell.
A further advantageous design consists in that the seal is formed as an inflatable tubular seal. This expandable seal only needs to withstand the internal pressure in the suction-pressure bell or in the narrow gap between the suction-pressure bell and the upper clamping frame and is not subjected to any pressing forces, as is the case with the known apparatuses.
The suction-pressure bell can preferably be vertically displaced by a drive device in order to permit demoulding to be carried out in particular in the case of positive moulds.
In a further design of the invention, the lower part can be moved together with the mould and the lower clamping frame against the suction-pressure bell by a drive device. The lower part and the mould are able to be connected to a negative pressure source.
For easier handling of the upper clamping frame, the upper clamping frame is able to be held in position on the suction-pressure bell by hydraulically or pneumatically activatable working cylinders.